This invention relates to microwave filters and more particularly to ferrite resonator filters.
The use of ferrite resonator filters is well established in the art. Previous filters consisted of wire loop coupling structures formed in a semicircular shape around ferrite cores orthogonal to each other. When subjected to a bias magnetic field external to both loops, these loops magnetically coupled to the ferrite, which resonated at the magnetically induced frequency. The loops were coupled to each other through the resonance of the common ferrite resonator. In multi-stage filters, this construction method is susceptible to variations in the manufacturing process, as well as from changes induced by mechanical or environmental stress. In the prior art, alignment of the loops was critical to achieve acceptable filter performance. An improved filter structure is desirable to minimize the need for alignment of the loops when the filter is subjected to an external bias magnetic field.
According to the invention, a ferrite resonator filter for use in connection with an external bias magnetic field provides a simple and reliable multi-resonant microstrip coupling filter structure wherein a metal circuit block is interposed between two substrates such that the circuit block is in contact with the sides of the substrates containing microstrip conductors and wherein the circuit block has orifices for receiving ferrite spheres.
The structure is less susceptible to variations in the manufacturing process, as well as to changes induced by mechanical or environmental stress. Additionally, the present invention allows for an assembly process that can be easily automated, reducing further manufacturing induced variation.
In a first aspect, the present invention provides a filter comprising two substrates made of dielectric material, a metal circuit block and ferrite spheres. The substrates have a conductive ground plane on one face and microstrip conductors on the opposite face. The microstrip conductors are positioned on the substrates so that they are orthogonal to one another and to the external bias magnetic field exerted on the filter. The first end of the first microstrip is connected to a microwave source while the second end is connected to ground. In contrast, the first end of the last microstrip is grounded while the second end of the last microstrip is connected to a microwave load. Both ends of the intermediate microstrip conductors are grounded. The metal circuit block is interposed between the two substrates such that the circuit block is in contact with the sides of the substrates containing microstrip conductors. Additionally, the circuit block has orifices for receiving the ferrite spheres.
In another aspect, the invention provides a filter comprising a single substrate with conductors on both faces of the substrate. Ferrites are then suspended from above or below the substrate.
In another aspect, the invention provides a filter comprising a single substrate with a conductor on the top side and a conductive ground plane on the bottom side. Ferrites are suspended above the substrate.